Miniature, electric power drawing elements, such as integrated circuits, semiconductors and the like, included within a larger host device typically have many electrically conductive pads. Each conductive pad requires electrical contact with one or more corresponding conductive pads on the same element or associated elements of the host device to form an electrically conductive connection between the pads. The electrically conductive connection is provided by a segment of fine, electrically conductive wire which extends between the pads and is bonded to a bonding site on each of the pads. Wire bonding is an automated operation performed during assembly of the host device using a machine having a high degree of speed and precision. The wire bonding operation is cyclic, comprising many bonding cycles integrated into a continuous unitary process. Each bonding cycle of the process comprises a number of discrete tasks performed sequentially by specific components of the wire bonding machine.
In a simple case where an electrical connection is required between only two pads, the bonding cycle comprises the tasks of retaining an element or elements including the pad pair, conveying a bonding wire from a wire supply to the pad pair, positioning the wire at the first bonding site on the first pad of the pair, bonding the wire to the first bonding site, repositioning the wire at the second bonding site on the second pad of the pair, bonding the wire to the second bonding site, and breaking off the wire at the second bonding site to provide a wire segment which electrically connects the first and second pads. These tasks are repeated in the next bonding cycle at another pad pair. For more complex bonding cycles where electrical connection is required between a string of three or more pads, the wire is positioned and bonded to the bonding site on each pad of the string in series. The wire is not broken off until the bond is completed at the final bonding site to provide a wire segment which electrically connects every pad of the string.
The task of conveying the bonding wire from the wire supply to the bonding site is critical to the performance of the wire bonding machine because any damage due to contamination, deformation, blemishing, or the like inflicted on the surface of the wire during the conveyance task can diminish the integrity of the ensuing wire bond. Accordingly, considerable effort in the prior art is directed to providing non-damaging means for conveying the bonding wire. For example, U.S. Pat. No. 5,318,234, incorporated herein by reference, discloses a wire de-spooler which conveys the bonding wire from a wire supply spool to a downstream wire handler. The wire de-spooler includes a spool drive motor to pay out the bonding wire from the spool and a wire slacking device comprising an air nozzle, a bail and a guide spindle positioned at the wire de-spooler outlet, which forms and maintains a desired slack length in the bonding wire. In operation, the wire slacking device suspends the bonding wire between a contact point on the spool and a contact point on the guide spindle as the bonding wire is paid out from the spool. The air nozzle and bail are positioned on the wire path between the spool and guide spindle. When the bonding wire passes through the bail, the air nozzle, which is positioned immediately above the bail, directs a downward jet of pressurized air directly onto the bonding wire. The force of the air jet on the bonding wire operates against the static force of the spool and guide spindle to create a slack length in the bonding wire between the spool and guide spindle. Maintenance of the slack length desirably reduces the risk of wire damage as the wire is being de-spooled. Maintenance of the slack length particularly reduces the risk of wire twisting during de-spooling which is extremely detrimental to subsequent wire bonding.
Although the above-described prior art wire slacking device may effectively prevent wire twisting, direct application of the air jet to the bonding wire in turbulent flow as well as contact between the bonding wire and the guide spindle or bail create a significant risk of damage to the bonding wire. The present invention recognizes a need for a wire conveyance system which controls the degree of slack in the bonding wire with a diminished risk of damage to the bonding wire. Therefore, it is an object of the present invention to provide an effective wire conveyance system. More particularly, it is an object of the present invention to provide a wire conveyance system having a device which effectively controls the desired degree of slack in the bonding wire as the bonding wire is conveyed between wire handlers. It is another object of the present invention to provide such a wire slacking device which controls the rate at which the wire is conveyed between wire handlers. It is yet another object of the present invention to provide such a wire slacking device which operates in a non-damaging manner with respect to the bonding wire. It is still another object of the present invention to provide such a wire slacking device which operates in a contactless manner with respect to the bonding wire and wire slacking device. These objects and others are accomplished in accordance with the invention described hereafter.